6-mercaptopurine, which is currently used as an antitumor agent, was discovered by Hitchings et al. in 1948 during studies on antimetabolites and synthesized by Elion et al. in 1952 from hypoxanthine. Clarke and Law et al. experimentally confirmed its antitumor effect in 1953, Burchenal et al. applied it clinically for the first time in 1953, and its superior effects were not only recognized in childhood acute leukemia, but also in adult acute leukemia. For that reason, among the numerous known chemotherapeutic agents for leukemia, it was the first substance to be called an anti-leukemic drug. Additionally, it is also used in an early stage and late stage of chronic leukemia, and is known to be effective.
6-mercaptopurine is a orally-effective nucleic acid antimetabolite for treating leukemia, and has been used to ameliorate subjective and objective symptoms of acute leukemia and chronic myeloid leukemia. However, it is known to have side effects including severe myelosuppression, liver dysfunction and hypersensitivity, and when considering patients' life of quality (QOL), they are issues that must be solved.
Moreover, the agent has also been used as an antimetabolite immunosuppressant, and has been used in Predonine non-responders and in cases where it is difficult to reduce the amount of or to go off Predonine. Since it takes three to four months before the effects are apparent, there is a need to watch for side effects. Further, due to myelosuppression as a side effect, infections tend to occur and renal dysfunction develops easily. Additionally, since resistance is weakened, all kinds of bacterial infections including colds occur easily. There is also a possibility for various complications to occur in conjunction.    Non-patent document 1: Chemistry & Biology, Vol. 11, 135-146, January 2004.    Patent document 1: WO 99/28321